This application claims the priority of German Application No. 100 13 430.0, filed Mar. 17, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a process for joining components made from case-hardened steel to one another or to components made from cast iron.
In modern production technology, it is of great interest to join components made from case-hardened steel to one another or to components made from cast iron. First, in terms of manufacturing technology it is often more expedient to initially produce two separate components and then to join them together. Second, the demands imposed on the component to be produced often cannot be fulfilled by one material.
By way of example, these problems arise in the automotive sector during the production of transmissions, e.g., in a differential gear the differential casing is produced from cast iron, on account of its geometrically complicated configuration. The ring gear connected thereto generally consists of case-hardened steel, so that at its surface it has a high strength and is wear-resistant, while having a relatively soft core material. The term case-hardened steel is understood as meaning steels which have a high surface hardness which is formed as a result of the workpiece being annealed in carbon-dominating media followed by quenching. As a result of the annealing operation, the surface layer of the workpieces is enriched with carbon and is hardened by a quenching operation from the annealing temperature.
In known differential gears, the connection between differential casing and ring gear is produced by screws. A screw connection has the drawback that a solid flange is required on the differential casing. Construction space is required for this flange and for the screw head and assembly. In addition, a minimum thread depth is required to achieve a secure connection. Therefore, the width of the ring gear must be able to accommodate the shank of the screw. Furthermore, the dimensions of the ring gear must be selected in such a way that it satisfies stability requirements, in particular because the plane of loading from the forces to be transmitted runs through the screw thread. These design boundary conditions mean that the ring gear must be of a minimum size. This minimum size, as well as the solid flange and the screw heads, have an adverse effect on the weight of the differential gear and therefore on production costs.
On account of the high carbon contents both in the cast iron and on the surface of the case-hardened steel, it is not possible to weld components of this type to one another. At the concentrations which result with these combinations of materials, the carbon in the molten material, during the rapid cooling after welding, forms brittle microstructural constituents which have an adverse effect on the quality of the weld seam and may lead to cracks forming.
Although WO 99/58287 A1 has disclosed a process in which a case-hardened ring gear is welded to a differential casing made from cast iron with spheroidal graphite. To do this, the surfaces which are to be welded on the otherwise fully machined components, prior to welding, are at least partially abraded for weld preparation, so that a narrow groove is formed where the welding is to be carried out. This machining step means that in the case-hardened ring gear the surface is removed in the region of the joint. This is the region of the component which has the highest carbon content. Since in a case-hardened steel the proportion of carbon falls very considerably at increasing distance from the surface, making the groove leads to an extreme reduction in the carbon content at the joint, with the result that the above-described problems involved in welding materials with high carbon contents are reduced considerably. In the differential gear described in WO 99/58287 A1, the welding is carried out with a welding wire being supplied continuously.
A drawback of this process is that the parts which are to be joined have to be prepared for the welding. The surfaces which are to be welded are at least partially abraded. This preparation represents an additional working step which, in the case of case-hardened steel, is associated with high machining costs, on account of the high strength.
In the known gear, the surfaces to be welded comprise two regions: a groove region and a centering region which is arranged beneath the groove region and at which the ring gear and differential casing abut against one another. After the welding, shrinkage processes cause this centering region to act as a notch on the weld seam, which affects the quality of this seam.
Furthermore, the continuous supply of welding wire during the welding operation represents a drawback, since this requires complex positioning and control of the welding wire feed rate. In the case of interference with these parameters, the filler is not uniformly distributed over the entire height and length of the weld seam. Particularly in the weld route, optimum mixing of the filler with the molten metal is not ensured.
In view of this background, the present invention is based on the object of providing a process for joining components made from case-hardened steel to one another or to components made from cast iron which is simple in terms of production technology and is inexpensive.
According to the present invention, this object is achieved by a process in which the components which are to be joined are welded together using a nickel-containing filler without being specially prepared for the welding operation (i.e. without at least part of the joining surfaces which are to be welded being abraded).
The process according to the present invention has the advantage that no weld preparation, which is highly complex in particular with case-hardened steels on account of the high strength of the surface, is required. The components can be welded together without any treatment of the joints, simply using a nickel-containing filler, for example pure nickel or X10CrNiTi 18 9. In the solidified weld seam, the nickel of the filler forms a buffer between the brittle microstructural constituents which form when the molten material solidifies and thus prevents cracks from forming in the weld seam. In this way, at least one machining step is saved, thus simplifying production and therefore making it more expedient. Particularly for series manufacture with high numbers, this advantage makes its presence felt in terms of costs.
It has proven particularly advantageous to add the filler in the form of a foil which is approximately 0.1 to 0.3 mm thick. This foil is laid between the components to be joined prior to the welding. The foil covers the entire area of the joint between the components. A complex device for supplying filler in wire form during welding is not required. Further, the filler is uniformly available over the entire height and length of the joint, leading to uniform mixing of the filler in the molten material and therefore to a reproducible quality of the seam which is constant over the height and length, in particular in the seam route.
Further, the exact position of the parts which are to be welded with respect to one another can be determined by the thickness of the foil. However, the presence of the filler which determines the quality of the welded joint can also be monitored by the position of the parts which are to be welded. In a preferred embodiment, the foil additionally serves as a spacer between the components which are to be joined, so that a gap is formed beneath the weld seam after welding, with the result that the weld seam is free of adverse effects from below (notch effects).
To achieve a high-quality weld seam, pure nickel has proven to be a particularly appropriate filler.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.